This invention relates to the synthesis, deprotection, and purification of enzymatic RNA or modified enzymatic RNA molecules in milligram to kilogram quantities with high biological activity.
The following is a brief history of the discovery and activity of enzymatic RNA molecules or ribozymes. This history is not meant to be complete but is provided only for understanding of the invention that follows. This summary is not an admission that all of the work described below is prior art to the claimed invention.
Prior to the 1970s it was thought that all genes were direct linear representations of the proteins that they encoded. This simplistic view implied that all genes were like ticker tape messages, with each triplet of DNA "letters" representing one protein "word" in the translation. Protein synthesis occurred by first transcribing a gene from DNA into RNA (letter for letter) and then translating the RNA into protein (three letters at a time). In the mid 1970s it was discovered that some genes were not exact, linear representations of the proteins that they encode. These genes were found to contain interruptions in the coding sequence which were removed from, or "spliced out" of, the RNA before it became translated into protein. These interruptions in the coding sequence were given the name of intervening sequences (or introns) and the process of removing them from the RNA was termed splicing. After the discovery of introns, two questions immediately arose: i) why are introns present in genes in the first place, and ii) how do they get removed from the RNA prior to protein synthesis? The first question is still being debated, with no clear answer yet available. The second question, how introns get removed from the RNA, is much better understood after a decade and a half of intense research on this question. At least three different mechanisms have been discovered for removing introns from RNA. Two of these splicing mechanisms involve the binding of multiple protein factors which then act to correctly cut and join the RNA. A third mechanism involves cutting and joining of the RNA by the intron itself, in what was the first discovery of catalytic RNA molecules.
Cech and colleagues were trying to understand how RNA splicing was accomplished in a single-celled pond organism called Tetrahymena thermophila. They had chosen Tetrahymena thermophila as a matter of convenience, since each individual cell contains over 10,000 copies of one intron-containing gene (the gene for ribosomal RNA). They reasoned that such a large number of intron-containing RNA molecules would require a large amount of (protein) splicing factors to get the introns removed quickly. Their goal was to purify these hypothesized splicing factors and to demonstrate that the purified factors could splice the intron-containing RNA in vitro. Cech rapidly succeeded in getting RNA splicing to work in vitro, but something unusual was going on. As expected, splicing occurred when the intron-containing RNA was mixed with protein-containing extracts from Tetrahymena, but splicing also occurred when the protein extracts were left out. Cech proved that the intervening sequence RNA was acting as its own splicing factor to snip itself out of the surrounding RNA. They published this startling discovery in 1982. Continuing studies in the early 1980's served to elucidate the complicated structure of the Tetrahymena intron and to decipher the mechanism by which self-splicing occurs. Many research groups helped to demonstrate that the specific folding of the Tetrahyrnena intron is critical for bringing together the parts of the RNA that will be cut and spliced. Even after splicing is complete, the released intron maintains its catalytic structure. As a consequence, the released intron is capable of carrying out additional cleavage and splicing reactions on itself (to form intron circles). By 1986, Cech was able to show that a shortened form of the Tetrahymena intron could carry out a variety of cutting and joining reactions on other pieces of RNA. The demonstration proved that the Tetrahymena intron can act as a true enzyme: i) each intron molecule was able to cut many substrate molecules while the intron molecule remained unchanged, and ii) reactions were specific for RNA molecules that contained a unique sequence (CUCU) which allowed the intron to recognize and bind the RNA. Zaug and Cech coined the term "ribozyme" to describe any ribonucleic acid molecule that has enzyme-like properties. Also in 1986, Cech showed that the RNA substrate sequence recognized by the Tetrahymena ribozyme could be changed by altering a sequence within the ribozyme itself. This property has led to the development of a number of site-specific ribozymes that have been individually designed to cleave at other RNA sequences. The Tetrahymena intron is the most well-studied of what is now recognized as a large class of introns, Group I introns. The overall folded structure, including several sequence elements, is conserved among the Group I introns, as is the general mechanism of splicing. Like the Tetrahymena intron, some members of this class are catalytic, i.e. the intron itself is capable of the self-splicing reaction. Other Group I introns require additional (protein) factors, presumably to help the intron fold into and/or maintain its active structure. While the Tetrahymena intron is relatively large, (413 nucleotides) a shortened form of at least one other catalytic intron (SunY intron of phage T4, 180 nucleotides) may prove advantageous not only because of its smaller size but because it undergoes self-splicing at an even faster rate than the Tetrahymena intron.
Ribonuclease P (RNAseP) is an enzyme comprised of both RNA and protein components which are responsible for converting precursor tRNA molecules into their final form by trimming extra RNA off one of their ends. RNAseP activity has been found in all organisms tested, but the bacterial enzymes have been the most studied. The function of RNAseP has been studied since the mid-1970s by many labs. In the late 1970s, Sidney Altman and his colleagues showed that the RNA component of RNAseP is essential for its processing activity; however, they also showed that the protein component also was required for processing under their experimental conditions. After Cech's discovery of self-splicing by the Tetrahymena intron, the requirement for both protein and RNA components in RNAseP was reexamined. In 1983, Altman and Pace showed that the RNA was the enzymatic component of the RNAseP complex. This demonstrated that an RNA molecule was capable of acting as a true enzyme, processing numerous tRNA molecules without itself undergoing any change. The folded structure of RNAseP RNA has been determined,. and while the sequence is not strictly conserved between RNAs from different organisms, this higher order structure is. It is thought that the protein component of the RNAseP complex may serve to stabilize the folded RNA in vivo At least one RNA position important both to substrate recognition and to determination of the cleavage site has been identified, however little else is known about the active site. Because tRNA sequence recognition is minimal, it is clear that some aspect(s) of the tRNA structure must also be involved in substrate recognition and cleavage activity. The size of RNAseP RNA (&gt;350 nucleotides), and the complexity of the substrate recognition, may limit the potential for the use of an RNAseP-like RNA in therapeutics. However, the size of RNAseP is being trimmed down (a molecule of only 290 nucleotides functions reasonably well). In addition, substrate recognition has been simplified by the recent discovery that RNAseP RNA can cleave small RNAs lacking the natural tRNA secondary structure if an additional RNA (containing a "guide" sequence and a sequence element naturally present at the end of all tRNAs) is present as well.
Symons and colleagues identified two examples of a self-cleaving RNA that differed from other forms of catalytic RNA already reported. Symons was studying the propagation of the avocado sunblotch viroid (ASV), an RNA virus that infects avocado plants. Symons demonstrated that as little as 55 nucleotides of the ASV RNA was capable of folding in such a way as to cut itself into two pieces. It is thought that in vivo self-cleavage of these RNAs is responsible for cutting the RNA into single genome-length pieces during viral propagation. Symons discovered that variations on the minimal catalytic sequence from ASV could be found in a number of other plant pathogenic RNAs as well. Comparison of these sequences revealed a common structural design consisting of three stems and loops connected by central loop containing many conserved (invariant from one RNA to the next) nucleotides. The predicted secondary structure for this catalytic RNA reminded the researchers of the head of a hammer consisting of three double helical domains, stems I, II and III and a catalytic core (FIGS. 1 and 2a); thus it was named as such. Uhlenbeck was successful in separating the catalytic region of the ribozyme from that of the substrate. Thus, it became possible to assemble a hammerhead ribozyme from 2 (or 3) small synthetic RNAs. A 19-nucleotide catalytic region and a 24-nucleotide substrate, representing division of the hammerhead domain, along the axes of stems I and II (FIG. 2b) were sufficient to support specific cleavage. The catalytic domain of numerous hammerhead ribozymes have now been studied by both the Uhlenbeck and Symons groups with regard to defining the nucleotides required for specific assembly and catalytic activity and determining the rates of cleavage under various conditions.
Haseloff and Gerlach showed it was possible to divide the domains of the hammerhead ribozyme in a different manner, division of the hammerhead domain along the axes of stems I and III (FIG. 2c). By doing so, they placed most of the required sequences in the strand that didn't get cut (the ribozyme) and only a required UH where H=C, A, U in the strand that did get cut (the substrate). This resulted in a catalytic ribozyme that could be designed to cleave any UH RNA sequence embedded within a longer "substrate recognition" sequence. The specific cleavage of a long mRNA, in a predictable manner using several such hammerhead ribozymes, was reported in 1988. A further development was the division of the catalytic hammerhead domain along the axes of stems III and II (FIG. 2d, Jeffries and Symons, Nucleic Acids. Res. 1989, 17, 1371-1377.)
One plant pathogen RNA (from the negative strand of the tobacco ringspot virus) undergoes self-cleavage but cannot be folded into the consensus hammerhead structure described above. Bruening and colleagues have independently identified a 50-nucleotide catalytic domain for this RNA. In 1990, Hampel and Tritz succeeded in dividing the catalytic domain into two parts that could act as substrate and ribozyme in a multiple-turnover, cutting reaction (FIG. 3). As with the hammerhead ribozyme, the hairpin catalytic portion contains most of the sequences required for catalytic activity while only a short sequence (GUC in this case) is required in the target. Hampel and Tritz described the folded structure of this RNA as consisting of a single hairpin and coined the term "hairpin" ribozyme (Bruening and colleagues use the term "paperclip" for this ribozyme motif, see, FIG. 3). Continuing experiments suggest an increasing number of similarities between the hairpin and hammerhead ribozymes in respect to both binding of target RNA and mechanism of cleavage. At the same time, the minimal size of the hairpin ribozyme is still 50-60% larger than the minimal hammerhead ribozyme.
Hepatitis Delta Virus (HDV) is a virus whose genome consists of single-stranded RNA. A small region (.about.80 nucleotides, FIG. 4) in both the genomic RNA, and in the complementary anti-genomic RNA, is sufficient to support self-cleavage. As the most recently discovered ribozyme, HDV's ability to self-cleave has only been studied for a few years, but is interesting because of its connection to a human disease. In 1991, Been and Perrotta proposed a secondary structure for the HDV RNAs that is conserved between the genomic and anti-genomic RNAs and is necessary for catalytic activity. Separation of the HDV RNA into "ribozyme" and "substrate" portions has recently been achieved by Been, but the rules for targeting different substrate RNAs have not yet been determined fully (see, FIG. 4). Been has also succeeded in reducing the size of the HDV ribozyme to .about.60 nucleotides.
The Table I lists some of the characteristics of the ribozymes discussed above.
Ribozymes are RNA molecules having an enzymatic activity which is able to repeatedly cleave other separate RNA molecules in a nucleotide base sequence specific manner. It is said that such enzymatic RNA molecules can be targeted to virtually any RNA transcript and efficient cleavage has been achieved in vitro. Kim et al., 84 Proc. Nat. Acad. of Sci. USA 8788, 1987; Haseloff and Gerlach, 334 Nature 585, 1988; Cech, 260 JAMA 3030, 1988; and Jefferies et al., 17 Nucleic Acid Research 1371, 1989.
Six basic varieties of naturally-occurrinig enzymatic RNAs are known presently. Each can catalyze the hydrolysis of RNA phosphodiester bonds in trans (and thus can cleave other RNA molecules) under physiological conditions. Table I summarizes some of the characteristics of these ribozymes. In general, enzymatic nucleic acids act by first binding to a target RNA. Such binding occurs through the target binding portion of a enzymatic nucleic acid which is held in close proximity to an enzymatic portion of the molecule that acts to cleave the target RNA. Thus, the enzymatic nucleic acid first recognizes and then binds a target RNA through complementary base-pairing, and once bound to the correct site, acts enzymatically to cut the target RNA. Strategic cleavage of such a target RNA will destroy its ability to direct synthesis of an encoded protein. After an enzymatic nucleic acid has bound and cleaved its RNA target, it is released from that RNA to search for another target and can repeatedly bind and cleave new targets.
By "enzymatic RNA molecule" it is meant an RNA molecule which has complementarity in a substrate binding region to a specified mRNA target, and also has an enzymatic activity which is active to specifically cleave that mRNA. That is, the enzymatic RNA molecule is able to intermolecularly cleave mRNA and thereby inactivate a target mRNA molecule. This complementarity functions to allow sufficient hybridization of the enzymatic RNA molecule to the target RNA to allow the cleavage to occur. One hundred percent complementarity is preferred, but complementarity as low as 50-75% may also be useful in this invention. For in vivo treatment, complementarity between 30 and 45 bases is preferred; although lower numbers are also useful.
By "complementary" is meant a nucleotide sequence that can form hydrogen bond(s) with other nucleotide sequence by either traditional Watson-Crick or other non-traditional types (for example Hoogsteen type) of base-paired interactions.
The enzymatic nature of a ribozyme is advantageous over other technologies, such as antisense technology (where a nucleic acid molecule simply binds to a nucleic acid target to block its translation) since the concentration of ribozyme necessary to affect a therapeutic treatment is lower than that of an antisense oligonucleotide. This advantage reflects the ability of the ribozyme to act enzymatically. Thus, a single ribozyme molecule is able to cleave many molecules of target RNA. In addition, the ribozyme is a highly specific inhibitor, with the specificity of inhibition depending not only on the base pairing mechanism of binding to the target RNA, but also on the mechanism of target RNA cleavage. Single mismatches, or base-substitutions, near the site of cleavage can completely eliminate catalytic activity of a ribozyme. Similar mismatches in antisense molecules do not prevent their action (Woolf, T. M., et al., 1992, Proc. Natl. Acad. Sci. USA, 89, 7305-7309). Thus, the specificity of action of a ribozyme is greater than that of an antisense oligonucleotide binding the same RNA site.
In preferred embodiments of this invention, the enzymatic nucleic acid molecule is formed in a hammerhead or hairpin motif, but may also be formed in the motif of a hepatitis delta virus, group I intron or RNaseP RNA (in association with an RNA guide sequence) or Neurospora VS RNA. Examples of such hammerhead motifs are described by Rossi et al., 1992, Aids Research and Human Retroviruses 8, 183, of hairpin motifs by Hampel et al., EPA 0360257, Hampel and Tritz, 1989 Biochemistry 28, 4929, and Hampel et al., 1990 Nucleic Acids Res. 18, 299, and an example of the hepatitis delta virus motif is described by Perrotta and Been, 1992 Biochemistry 31, 16; of the RNaseP motif by Guerrier-Takada et al., 1983 Cell 35, 849, Neurospora VS RNA ribozyme motif is described by Collins (Saville and Collins, 1990 Cell 61, 685-696; Saville and Collins, 1991 Proc. Natil. Acad, Sci. USA 88, 8826-8830; Collins and Olive, 1993 Biochemistry 32, 2795-2799) and of the Group I intron by Cech et al., U.S. Pat. No. 4,987,071. These specific motifs are not limiting in the invention and those skilled in the art will recognize that all that is important in an enzymatic nucleic acid molecule of this invention which is complementary to one or more of the target gene RNA regions, and that it have nucleotide sequences within or surrounding that substrate binding site which impart an RNA cleaving activity to the molecule.
Enzymatic nucleic acids act by first binding to a target RNA (or DNA, see Cech U.S. Pat. No. 5,180,818). Such binding occurs through the target binding portion of an enzymatic nucleic acid which is held in close proximity to an enzymatic portion of molecule that acts to cleave the target RNA. Thus, the enzymatic nucleic acid first recognizes and then binds a target RNA through complementary base-pairing, and once bound to the correct site, acts enzymatically to cut the target RNA. Cleavage of such a target RNA will destroy its ability to direct synthesis of an encoded protein. After an enzymatic nucleic acid has bound and cleaved its RNA target it is released from that RNA to search for another target and can repeatedly bind and cleave new targets.
The invention provides a method for producing a class of enzymatic cleaving agents or antisense molecules which exhibit a high degree of specificity for the RNA or DNA of a desired target. The enzymatic nucleic acid or antisense molecule is preferably targeted to a highly conserved sequence region of a target such that specific treatment of a disease or condition can be provided with a single enzymatic nucleic acid. Such, nucleic acid molecules can be delivered exogenously to specific cells as required. In the preferred hammerhead motif the small size (less than 60 nucleotides, preferably between 30-40 nucleotides in length) of the molecule allows the cost of treatment to be reduced compared to other ribozyme motifs.
Synthesis of nucleic acids greater than 100 nucleotides in length is difficult using automated methods, and the therapeutic cost of such molecules is prohibitive. In this invention, small enzymatic nucleic acid motifs (e.g., of the hammerhead structure) are used for exogenous delivery. The simple structure of these molecules increases the ability of the enzymatic nucleic acid to invade targeted regions of the mRNA structure. Unlike the situation when the hammerhead structure is included within longer transcripts, there are no non-enzymatic nucleic acid flanking sequences to interfere with correct folding of the enzymatic nucleic acid structure or with complementary regions.
Generally, RNA is synthesized and purified by methodologies based on: tetrazole to activate the RNA amidite, NH.sub.4 OH to remove the exocyclic amino protecting groups, tetra-n-butylammonium fluoride (TBAF) to remove the 2'-OH alkylsilyl protecting groups, and gel purification and analysis of the deprotected RNA. In particular this applies to, but is not limited to, a certain class of RNA molecules, ribozymes. These may be formed either chemically or using enzymatic methods. Examples of the chemical synthesis, deprotection, purification and analysis procedures are provided by Usman et al., 1987 J. American Chem. Soc., 109, 7845, Scaringe et al. Nucleic Acids Res. 1990, 18, 5433-5341, Perreault et al. Biochemistry 1991, 30 4020-4025, and Slim and Gait Nucleic Acids Res. 1991, 19, 1183-1188. Odai et al. FEBS Lett. 1990, 267, 150-152 describes a reverse phase chromatographic purification of RNA fragments used to form a ribozyme. All the above noted references are all hereby incorporated by reference herein.
The aforementioned chemical synthesis, deprotection, purification and analysis procedures are time consuming (10-15 m coupling times) and may also be affected by inefficient activation of the RNA amidites by tetrazole, time consuming (6-24 h) and incomplete deprotection of the exocyclic amino protecting groups by NH.sub.4 OH, time consuming (6-24 h), incomplete and difficult to desalt TBAF-catalyzed removal of the alkylsilyl protecting groups, time consuming and low capacity purification of the RNA by gel electrophoresis, and low resolution analysis of the RNA by gel electrophoresis.
Imazawa and Eckstein, 1979 J. Org. Chem., 12, 2039, describe the synthesis of 2'-amino-2'-deoxyribofuranosyl purines. They state that
"To protect the 2'-amino function, we selected the trifluoroacetyl group which can easily be removed."